The quality standards in the plastic manufacturing industries are permanently increasing. Therefore, plastic material has to be inspected and analyzed for different types of defects before it enters the final product. Contamination, defects or inhomogeneities down to a size of 50µm in the material have to be detected. This is achieved by a modular X-ray scanner and analysis equipment that can either be used continuously “at-line” or for sample testing. In this way, transparent, opaque, colored or black pellets and flakes as well as thick colored films and tapes can reliably be inspected and analyzed. Consequently, highest material purity and stable production processes can be assured.
 

Introduction: Rising quality demands and challenges in the plastic industry
 

For industrial production, highly pure and flawless plastic material is an important quality aspect. Due to the continuously increasing requirements, it is necessary to detect and monitor defects and impurities of decreasing size in plastic and intermediate products. Impurities of 50µm may already cause damage to production systems, such as to the crosshead of the extruder, or to the end products with high follow-up costs. An example can be taken from the cable industry. For the production of a high-voltage subsea cable, it is of crucial importance to use highly, or so called super clean, material. A contamination, which enters the cable during production, can lead to a massive failure [1] of the cable once it is positioned very deep under the sea level. It costs millions to repair a broken subsea cable. For the cable industry, it is especially important to detect metallic contaminations before the material is used for final production. Therefore, latest norms and standards, such as the Chinese standard IEC 62067 (for 150 – 500 kV) for high voltage cables, demand the exclusion of contamination from 75µm in the processed material [2]. However, not only the cable industry has these requirements. Also other industries, such as the medical, hose and tube or car industries, have to monitor and analyze the quality of their used material.
 
In the different processing steps for the production of plastic products, defects may repeatedly occur. This affects the processes of material producers as well as the compound and masterbatch producers, the processing industry, the recyclers and the whole supply chain. Accordingly, it is necessary to control and monitor the quality of the material used before it is further processed.
 
In an industrial process in the plastic manufacturing industry, it is possible to find many different types of possible defects, which enter the end product. These different types of defects can have different effects on the final product. Here we can distinguish between visual effects, but also effects on the functions of the end product as described, for example, in the cable or medical industries. Defects can also cause damages to high sophisticated processing machines such as an extruder or injection molding machines. Today, there are various systems and technologies available in order to inspect materials in laboratories or during production. Most of the technologies are based on optical film inspection technologies, where a batch sample is extruded to a film. [3] [4] [5]. There are several limitations to these optical inspection systems:
 

  • Optical inspection systems such as pellet inspection systems only inspect the surface of pellets and are not able to look inside the pellet. This applies especially for opaque and colored pellets.
  • Optical film inspection is only applicable for transparent, thin films. Colored pellets cannot be inspected through a film inspection and analysis system without accepting major drawbacks. The only possibility to inspect colored material through film inspection is to produce a very thin film. However, this means that only a very low quantity of pellets can be inspected. Therefore, the results of thin film inspections are not representative.

 

The limitations of the today’s optical inspection systems require new ways and technologies to meet fully the requirements of the plastic industry.
 
With new X-ray test technologies and methods production related parameters such as inhomogeneity, contamination and defects, material differences, cross contamination, etc. can be analyzed.
 

X-ray technology to inspect and analyze the inside of a plastic pellet or colored thick films
 

By the use of X-ray technology, both transparent as well as non-transparent plastics are inspected for contamination. The basic principle is the different attenuation of raw material and impurities or defects respectively. The attenuation (µ) of the X-rays is mainly determined upon the nuclear charge of elements as well as of the thickness of the material to be inspected [6]. It is proportional to the atomic number raised to the 3rd power (µ~Z3).
 
Plastics consist mainly of carbon (Z=6). Thus, they have only a very low attenuation. An iron contamination with comparatively strong attenuation (Z=26) can be detected clearly. An additive, for example titanium dioxide, also affects the attenuation. Titanium dioxide agglomerates significantly contrast in the dispersion with the surrounding material. This is possible because the titanium (Z=22) in the titanium dioxide strongly contrasts with the plastics.
 
With a specific developed X-ray camera system, it is possible to detect contamination or defects during continuous and discontinuous inspections, during film inspection or as an “at line” continues quality monitoring system. X-ray cameras are taking pictures of the plastic pellets or of the film, which are then processed by mathematical algorithm. The mathematical algorithm clearly identifies the difference in the attenuation, even for a contamination size down to 50µm. By combining the analytical results of the mathematical algorithm, the system clearly identifies for example contaminants in plastic pellets (Picture 1).
 
Pic_1_X_ray_analysis
Picture 1 The X-ray analysis shows two contaminants within a round pellet
 

Further possibilities with X-ray to monitor and optimize material quality
 

The production processes in the plastic industry are complex. Different materials are mixed together to receive different end products and there are many sources of contamination and other defects, which can enter the product. X-ray technology does not only detect metallic impurities in plastic pellets but gives also information on the condition of the overall production process. With X-ray, it is for example possible to identify cross-contamination of different plastic materials in one production process. This cross-contamination can be clearly visible, even if a plastic pellet has the same color, but a completely different attenuation (Picture 2). A main advantage of X-ray technology is that it is color independent.
 
Pic_2_Different_pellet_type_visible
Picture 2 Different pellet type is visible due to higher X-ray attenuation
 

Further tests show, that it is also possible to detect an organic contamination, which in this example was probably a piece of tissue as well as a small metallic contamination. It is also possible to see clearly that one pellet has an air hole inside, which could lead to the conclusion that some parameters in the production process need to be adapted (Picture 3).
 
Pic_3_Vacuoles_metallic_organic_contamination
Picture 3 Vacuoles, metallic and organic contamination
 

X-ray can also be used to detect agglomerations of additives in polyethylene pellets. The following example shows polyethylene with titanium oxide. X-ray was able to detect agglomerations of titanium oxide particles inside the plastic pellets (Picture 4). These agglomerations can lead to major problems when they are used in a further production process to manufacture final consumer products.
 
Pic_4_Agglomerates
Picture 4 Inhomogeneities/ agglomerations inside of the pellets
 

The determination of the correct degree of foaming or expansion is also a possible analysis, which can be realized by X-raying the respective pellets (Picture 5). In contrast, an optical inspection of the pellets would have given the information that all the pellets are flawless. However, an incorrect degree of foaming or expansion of the pellets causes major defects in the end product, which can be prevented by using X-ray.
 
Pic_5_Foaming
Picture 5 Incorrect degree of foaming or expansion inside of the pellet
 

In a further test, we used black pellets with different metallic contamination in order to prove the efficiency of X-ray technologies. In picture 6, different black polyethylene pellets are visible. In these pellets, we clearly identified contamination of the size of 50µm or 100µm.
 
Pic_6_Metallic_contamination_in_black_pellets
Picture 6 Metallic contamination in black pellets
 

With the X-ray analysis it is also possible to detect contaminants in flakes. The following picture shows small and major metallic contaminants in flakes (Picture 7). An optical analysis would not be able to provide these detailed results as the flakes are reflective and therefore would disturb the picture analysis of an optical camera. With the X-ray analysis system, it is possible to detect metallic contaminants and also classify their size. Due to this information it is possible to determine the quality degree of the flakes.
 
Pic_7_Flakes
Picture 7 Metallic contamination in flakes
 

The same principle can be used for colored thick film inspection and analysis. As already mentioned, optical film inspection needs transparent thin films and therefore cannot be used for an inspection and analysis such as shown in Picture 8. Here the film has a dark color and is several millimeters thick. An X-ray analysis system is able to detect smallest contaminants in the film and therefore gives information on the contamination level of the overall material.
 
Pic_8_Film
Picture 8 Contamination in colored thick films
 

Conclusion

 
Quality standards in the plastic manufacturing industry are constantly increasing. Contamination, inhomogeneities or any other defects need to be detected and analyzed. Today, even the smallest contamination need to be detected during the production process. With the X-ray technology, we are able to detect contamination of 50µm.
 
Different inspection and analysis requirements need a modular concept approach for the equipment. Such equipment can be either used as a discontinuous lab scanner for pellets and flakes or as a continuous quality monitoring device of a production line. In this case the equipment is installed as an “at-line” quality monitoring and analysis device and can be used for pellet, flake inspection as well. The following pictures demonstrate the modular concept of an X-ray scanning and analysis device.
 
Pic_9_transport_system
Picture 9 X-ray scanning and analysis equipment with conveyor belt for a continuous inspection flow (e.g. “at-line”)
 
Pic_10_Film_Tape
Picture 10 X-ray scanning and analysis equipment for film and tape inspection
 
Pic_11_Sample_Tray
Picture 11 X-ray scanning and analysis equipment for discontinuous inspection and analysis – sample test (pellets, flakes, etc.)
 
The concept is applicable for different material types as for instance TPE, TPU, etc. or any colored masterbatch or recycling product. The detection is independent of the color of the pellet or the color and the thickness of the film or tape. Therefore, the quality can be constantly monitored and, hence, significantly improved.
 

References
 
1. Mr. Omar Monajjed, High Voltage Technical Manager, LIBAN CABLES — NEXANS, Lebanon, Effect of impurities on electric field distribution in HV XLPE insulation, Polymers in Cables 2014, Philadelphia, USA
2. J. Kjellqvist, K.P. Pang, S. Miao, Dow Europe GMBH, Horgen, Switzerland, Dow Chemical (China) Co. Ltd., Shanghai, China, Performance Requirements to Assure Reliable HV and EHV Cables, China International Conference on Electricity Distribution (CICED 2010) Nanjing (20-23 Sep. 2010)
3. Tomra demonstrates optical sorter, https://www.plasticsnews.com/article/20131030/NEWS/131039997/tomra-demonstrates-optical-sorter#, (10/2013)
4. Laura Tarrach, OCS GmbH, Pellet Scanning in “Free Fall”, Kunststoffe international (12/2010)
5. Satake, Pellet Sorter PCS600PFD, https://www.satake-usa.com/images/Pellet_Sorter_Brochure.pdf (2014)
6. Robert Fosbinder, Denise Orth, Lippincott Williams & Wilkins, Essentials of Radiologic Science (01.02.2011)